Document Outline
- FEATURES
- APPLICATIONS
- GENERAL DESCRIPTION
- SPECIFICATIONS
-
-
-
-
-
-
-
-
- OUTPUT INTERFACE
- SETPOINT INTERFACE
- TEMPERATURE COMPENSATION OF OUTPUT VOLTAGE
- TEMPERATURE SENSOR
- MEASUREMENT MODE
- DEVICE CALIBRATION AND ERROR CALCULATION
- SELECTING CALIBRATION POINTS TO IMPROVE ACCURACY OVER A REDUCED RANGE
- VARIATION IN TEMPERATURE DRIFT FROM DEVICE TO DEVICE
- TEMPERATURE DRIFT AT DIFFERENT TEMPERATURES
- SETTING THE OUTPUT SLOPE IN MEASUREMENT MODE
- RESPONSE TIME CAPABILITY
- CONTROLLER MODE
- CHARACTERIZATION SETUPS AND METHODS
-
-
1 MHz 8 GHz, 60 dB
Logarithmic Detector/Controller
AD8318
Rev. 0
Information furnished by Analog Devices is believed to be accurate and reliable.
However, no responsibility is assumed by Analog Devices for its use, nor for any
infringements of patents or other rights of third parties that may result from its
use. Specifications subject to change without notice. No license is granted by
implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registered trademarks are the property of their respective
owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.326.8703
2003 Analog Devices, Inc. All rights reserved.
FEATURES
Wide bandwidth: 1 MHz to 8 GHz
High accuracy: 1.0 dB over 55 dB range (f < 5.8 GHz)
Stability over temperature: 0.5 dB
Low noise measurement/controller output VOUT
Pulse response time 10/12 ns (fall/rise)
Integrated temperature sensor
Small footprint CSP package
Power-down feature: <1.5 mW at 5 V
Single-supply operation: 5V @ 68 mA
Fabricated using high speed SiGe process
APPLICATIONS
RF transmitter PA setpoint control and level monitoring
RSSI measurement in base stations, WLAN, radar
GENERAL DESCRIPTION
The AD8318 is a demodulating logarithmic amplifier, capable of
accurately converting an RF input signal to a corresponding
decibel-scaled output voltage. It employs the progressive
compression technique over a cascaded amplifier chain, each
stage of which is equipped with a detector cell. The device can be
used in measurement or controller mode. The AD8318
maintains accurate log conformance for signals of 1 MHz to
6 GHz and provides useful operation to 8 GHz. The input range
is typically 60 dB (re: 50 ) with error less than 1 dB. The
AD8318 has a 10 ns response time that enables RF burst
detection to beyond 60 MHz. The device provides
unprecedented logarithmic intercept stability versus ambient
temperature conditions. A 2 mV/K slope temperature sensor
output is also provided for additional system monitoring. A
single supply of +5 V is required. Current consumption is
typically 68 mA. Power consumption decreases to <1.5 mW
when the device is disabled.
The AD8318 can be configured to provide a control voltage to a
VGA, such as a power amplifier or a measurement output, from
pin VOUT. Since the output can be used for controller
FUNCTIONAL BLOCK DIAGRAM
TEMP
SENSOR
GAIN
BIAS
SLOPE
DET
DET
DET
DET
INHI
INLO
I
V
VOUT
I
V
VSET
CLPF
TEMP
VPSI
ENBL
TADJ
VPSO
CMOP
CMIP
04853-
001
Figure 1.
applications, special attention has been paid to minimize
wideband noise. In this mode, the setpoint control voltage is
applied to VSET. The feedback loop through an RF
amplifier is closed via VOUT; the output of which regulates
the amplifier's output to a magnitude corresponding to V
SET
.
The AD8318 provides 0 V to 4.9 V output capability at the
VOUT pin, suitable for controller applications. As a
measurement device, VOUT is externally connected to
VSET to produce an output voltage V
OUT
that is a decreasing
linear-in-dB function of the RF input signal amplitude.
The logarithmic slope is nominally -25 mV/dB, but can be
adjusted by scaling the feedback voltage from VOUT to the
VSET interface. The intercept is +20 dBm (re: 50 , CW
input) using the INHI input. These parameters are very
stable against supply and temperature variations.
The AD8318 is fabricated on a SiGe bipolar IC process and
is available in a 4 mm 4 mm, 16-pin LFCSP package, for
the operating temperature range of 40
o
C to +85
o
C.
AD8318
Rev. 0 | Page 2 of 24
TABLE OF CONTENTS
Specifications ...................................................................................3
Absolute Maximum Ratings ..........................................................6
ESD Caution ................................................................................6
Pin Configuration and Functional Descriptions ........................7
Typical Performance Characteristics............................................8
General Description .................................................................... 11
Using the AD8318........................................................................ 12
Basic Connections ................................................................... 12
Enable ........................................................................................ 12
Input Signal Coupling ............................................................. 12
Output Interface....................................................................... 13
Setpoint Interface..................................................................... 13
Temperature Compensation of Output Voltage .................. 13
Temperature Sensor................................................................. 14
Measurement Mode ................................................................. 14
Device Calibration and Error Calculation............................ 15
Selecting Calibration Points to Improve Accuracy over a
Reduced Range ......................................................................... 16
Variation in Temperature Drift from Device to Device...... 17
Temperature Drift at Different Temperatures ...................... 17
Setting the Output Slope in Measurement Mode ................ 17
Response Time Capability ...................................................... 18
Controller Mode....................................................................... 18
Characterization Setups and Methods .................................. 20
Evaluation Board .......................................................................... 21
Outline Dimensions..................................................................... 23
Ordering Guide ........................................................................ 23
REVISION HISTORY
7/04--Revision 0: Initial Version
AD8318
Rev. 0 | Page 3 of 24
SPECIFICATIONS
V
P
= 5 V, C
LPF
= 220 pF, T
A
= +25C, 52.3 termination resistor at INHI, unless otherwise noted.
Table 1.
Parameter
Conditions
Min
Typ
Max
Unit
SIGNAL INPUT INTERFACE
INHI (Pin 14) and INLO (Pin 15)
Specified Frequency Range
0.001
8
GHz
DC Common-Mode Voltage
VPOS 1.8
V
MEASUREMENT MODE
VOUT (Pin 6) shorted to VSET (Pin 7), sinusoidal
input signal
f = 900 MHz
500 at TADJ to GND
Input Impedance
957 || 0.71
|| pF
1 dB Dynamic Range
T
A
= +25
C
57
dB
-40
C < T
A
< +85
C
48
dB
Maximum Input Level
1 dB Error
1
dBm
Minimum Input Level
1 dB Error
58
dBm
Slope
-26
24.5
-23
mV/dB
Intercept
19.5
22
24
dBm
Output Voltage--High Power In
P
IN
= 10 dBm
0.7
0.78
0.86
V
Output Voltage--Low Power In
P
IN
= 40 dBm
1.42
1.52
1.62
V
Temperature Sensitivity
P
IN
= 10 dBm
25
C T
A
+85C
+0.0011
dB/
C
-40
C T
A
+25C
+0.003
dB/
C
f = 1.9 GHz
500 at TADJ to GND
Input Impedance
523 || 0.68
|| pF
1 dB Dynamic Range
T
A
= +25
C
57
dB
-40
C < T
A
< +85
C
50
dB
Maximum Input Level
1 dB Error
2
dBm
Minimum Input Level
1 dB Error
59
dBm
Slope
-27
24.4
-22
mV/dB
Intercept
17
20.4
24
dBm
Output Voltage--High Power In
P
IN
= 10 dBm
0.63
0.73
0.83
V
Output Voltage--Low Power In
P
IN
= 35 dBm
1.2
1.35
1.5
V
Temperature Sensitivity
P
IN
= 10 dBm
25
C T
A
+85C
+0.0011
dB/
C
40
C T
A
+25C
+0.0072
dB/
C
f = 2.2 GHz
500 at TADJ to GND
Input Impedance
391 || 0.66
|| pF
1 dB Dynamic Range
T
A
= +25
C
58
dB
-40
C < T
A
< +85
C
50
dB
Maximum Input Level
1 dB Error
2
dBm
Minimum Input Level
1 dB Error
60
dBm
Slope
-28
24.4
-21.5
mV/dB
Intercept
15
19.6
25
dBm
Output Voltage--High Power In
P
IN
= 10 dBm
0.63
0.73
0.84
V
Output Voltage--Low Power In
P
IN
= 35 dBm
1.2
1.34
1.5
V
Temperature Sensitivity
P
IN
= 10 dBm
25
C T
A
+85C
-0.0005
dB/
C
40
C T
A
+25C
+0.0062
dB/
C
AD8318
Rev. 0 | Page 4 of 24
Parameter
Conditions
Min
Typ
Max
Unit
f = 3.6 GHz
51 at TADJ to GND
Input Impedance
119 || 0.7
|| pF
1 dB Dynamic Range
T
A
= +25
C
58
dB
-40
C < T
A
< +85
C
42
dB
Maximum Input Level
1 dB Error
2
dBm
Minimum Input Level
1 dB Error
60
dBm
Slope
24.3
mV/dB
Intercept
19.8
dBm
Output Voltage--High Power In
P
IN
= 10 dBm
0.717
V
Output Voltage--Low Power In
P
IN
= 40 dBm
1.46
V
Temperature Sensitivity
P
IN
= 10 dBm
25
C T
A
+85C
+0.0022
dB/
C
40
C T
A
+25C
+0.004
dB/
C
f = 5.8 GHz
1000 at TADJ to GND
Input Impedance
33 || 0.59
|| pF
1 dB Dynamic Range
T
A
= +25
C
57
dB
-40
C < T
A
< +85
C
48
dB
Maximum Input Level
1 dB Error
1
dBm
Minimum Input Level
1 dB Error
58
dBm
Slope
24.3
mV/dB
Intercept
25
dBm
Output Voltage--High Power In
P
IN
= 10 dBm
0.86
V
Output Voltage--Low Power In
P
IN
= 40 dBm
1.59
V
Temperature Sensitivity
P
IN
= 10 dBm
25
C T
A
+85C
+0.0033
dB/
C
40
C T
A
+25C
+0.0069
dB/
C
f = 8.0 GHz
500 at TADJ to GND
3 dB Dynamic Range
T
A
= +25
C
60
dB
-40
C < T
A
< +85
C
58
dB
Maximum Input Level
3 dB Error
3
dBm
Minimum Input Level
3 dB Error
55
dBm
Slope
23
mV/dB
Intercept
37
dBm
Output Voltage--High Power In
P
IN
= 10 dBm
1.06
V
Output Voltage--Low Power In
P
IN
= 40 dBm
1.78
V
Temperature Sensitivity
P
IN
= 10 dBm
25
C T
A
+85C
+0.028
dB/
C
40
C T
A
+25C
-0.0085
dB/
C
OUTPUT INTERFACE
VOUT (Pin 6)
Voltage Swing
VSET = 0 V; RFIN = 10 dBm, no load
1
4.9
V
VSET = 2.1 V; RFIN = 10 dBm, no load
1
25
mV
Output Current Drive
VSET = 1.5 V, RFIN = 50 dBm
60
mA
Small Signal Bandwidth
RFIN = -10 dBm; From CLPF to VOUT
600
MHz
Output Noise
RF Input = 2.2 GHz, 10 dBm, f
NOISE
= 100 kHz,
CLPF = 220 pF
90
nV/
Hz
AD8318
Rev. 0 | Page 5 of 24
Parameter
Conditions
Min
Typ
Max
Unit
Fall Time
Input Level = off to 10 dBm, 90% to 10%
10
ns
Rise Time
Input Level = 10 dBm to off, 10% to 90%
12
ns
VSET INTERFACE
VSET (Pin 7)
Nominal Input Range
RFIN = 0 dBm; measurement mode
2
0.5
RFIN = 65 dBm; measurement mode
2
2.1
V
Logarithmic Scale Factor
0.04
dB/mV
Bias Current Source
RFIN = -10 dBm; VSET = 2.1 V
2.5
A
TEMPERATURE REFERENCE
TEMP (Pin 13)
Output Voltage
T
A
= 25
C, R
L
= 10 k
0.57 0.6 0.63
V
Temperature Slope
40
C T
A
+85C, R
L
= 10 k
2
mV/
C
Current Source/Sink
T
A
= 25
C
10/0.1
mA
POWER-DOWN INTERFACE
ENBL (Pin 16)
Logic Level to Enable Device
1.7
V
ENBL Current When Enabled
ENBL = 5 V
<1
A
ENBL Current When Disabled
ENBL = 0 V; Sourcing
15
A
POWER INTERFACE
VPSI (Pins 3, 4), VPSO (Pin 9)
Supply Voltage
4.5
5
5.5
V
Quiescent Current
ENBL = 5 V
50
68
52
mA
vs. Temperature
40
C T
A
+85C
68
mA
Supply Current when Disabled
ENBL = 0 V, Total Currents for VPSI and VPSO
260
A
vs. Temperature
40
C T
A
+85C
350
A
1
Controller mode
2
(Gain = 1) For other gains, see Measurement Mode section of the data sheet.
AD8318
Rev. 0 | Page 6 of 24
ABSOLUTE MAXIMUM RATINGS
Table 2.
Parameter Rating
Supply Voltage: VPSO, VPSI
ENBL, VSET Voltage
Input Power (Single-ended, re: 50
)
Internal Power Dissipation
JA
1
Maximum Junction Temperature
Operating Temperature Range
Storage Temperature Range
Lead Temperature Range
5.7 V
0 to VP
12 dBm
0.73 W
55
C/W
125
C
40
C to +85C
65
C to +150C
260
C
1
With package die paddle soldered to thermal pads with vias connecting to inner
and bottom layers
Stresses above those listed under Absolute Maximum
Ratings may cause permanent damage to the device. This is
a stress rating only; functional operation of the device at
these or any other conditions above those indicated in the
operational section of this specification is not implied.
Exposure to absolute maximum rating conditions for
extended periods may affect device reliability.
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on
the human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.
AD8318
Rev. 0 | Page 7 of 24
PIN CONFIGURATION AND FUNCTIONAL DESCRIPTIONS
04853-002
12
CMIP
11
CMIP
10
TADJ
9
VPSO
1
CMIP
2
CMIP
3
VPSI
4
VPSI
13
TEMP
8
CMOP
14
INHI
7
VSET
15
INLO
6
VOUT
16
ENBL
5
CLPF
AD8318
Figure 2. 16-Lead Lead Frame Chip Scale Package (LFCSP)
Table 3. Pin Function Descriptions
Pin No.
Mnemonic
Function
1, 2, 11, 12
CMIP
Device Common (Input System Ground).
3, 4, 9
VPSI, VPSO
Positive Supply Voltage for the Device Input System: 4.5 V to 5.5 V (voltage on all pins should be equal).
5
CLPF
Loop Filter Capacitor.
6
VOUT
Measurement and Controller Output.
7
VSET
Setpoint Input for Controller Mode, or Feedback Input for Measurement Mode.
8
CMOP
Device Common (Output System Ground).
10
TADJ
Temperature Compensation Adjustment.
13
TEMP
Temperature Sensor Output.
14 INHI
RF Input. Nominal input range: -60 dBm to 0 dBm re: 50
; ac-coupled RF input.
15
INLO
RF Common for INHI;
ac-coupled RF common.
16
ENBL
Device Enable. Connect to VPSI for normal operation. Connect pin to ground for disable mode.
Paddle
Internally Connected to CMIP, Solder to Ground.
AD8318
Rev. 0 | Page 8 of 24
TYPICAL PERFORMANCE CHARACTERISTICS
V
P
= 5 V, T = +25C, 40C, +85C; C
LPF
= 220 pF; T
ADJ
= 500 ; unless otherwise noted. Colors:
+25C Black; 40C Blue; +85C Red
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
65
55
45
35
25
15
5
5
15
04853-003
P
IN
(dBm)
V
OUT
(V
)
2.0
2.0
1.6
1.2
0.8
0.4
0
0.4
0.8
1.2
1.6
E
RROR (dB)
Figure 3. V
OUT
and Log Conformance vs. Input Amplitude at 900 MHz,
Typical Device
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
65
55
45
35
25
15
5
5
15
04853-004
P
IN
(dBm)
V
OUT
(V
)
2.0
2.0
1.6
1.2
0.8
0.4
0
0.4
0.8
1.2
1.6
E
RROR (dB)
Figure 4. V
OUT
and Log Conformance vs. Input Amplitude at 1.9 GHz,
Typical Device
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
65
55
45
35
25
15
5
5
15
04853-005
P
IN
(dBm)
V
OUT
(V
)
2.0
2.0
1.6
1.2
0.8
0.4
0
0.4
0.8
1.2
1.6
E
RROR (dB)
Figure 5. V
OUT
and Log Conformance vs. Input Amplitude at 3.6 GHz,
Typical Device, T
ADJ
= 51
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
04853-006
P
IN
(dBm)
V
OUT
(V
)
2.0
2.0
1.6
1.2
0.8
0.4
0
0.4
0.8
1.2
1.6
E
RROR (dB)
65
55
45
35
25
15
5
5
15
Figure 6. V
OUT
and Log Conformance vs. Input Amplitude at 5.8 GHz,
Typical Device, T
ADJ
= 1000
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
65
55
45
35
25
15
5
5
15
04853-007
P
IN
(dBm)
V
OUT
(V
)
2.0
2.0
1.6
1.2
0.8
0.4
0
0.4
0.8
1.2
1.6
E
RROR (dB)
Figure 7. V
OUT
and Log Conformance vs. Input Amplitude at 2.2 GHz,
Typical Device
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
65
55
45
35
25
15
5
5
04853-008
P
IN
(dBm)
V
OUT
(V
)
4.5
4.5
3.6
2.7
1.8
0.9
0
0.9
1.8
2.7
3.6
E
RROR (dB)
Figure 8. V
OUT
and Log Conformance vs. Input Amplitude at 8 GHz,
Typical Device
AD8318
Rev. 0 | Page 9 of 24
2.0
2.0
1.6
1.2
0.8
0.4
0
0.4
0.8
1.2
1.6
65
55
45
35
25
15
5
5
15
04853-009
P
IN
(dBm)
E
RROR (dB)
Figure 9. Distribution of Error over Temperature after Ambient
Normalization vs. Input Amplitude at 900 MHz for at least 70 Devices
2.0
2.0
1.6
1.2
0.8
0.4
0
0.4
0.8
1.2
1.6
65
55
45
35
25
15
5
5
15
04853-010
P
IN
(dBm)
E
RROR (dB)
Figure 10. Distribution of Error at Temperature after Ambient
Normalization vs. Input Amplitude at 1900 MHz for at least 70 Devices
2.0
2.0
1.6
1.2
0.8
0.4
0
0.4
0.8
1.2
1.6
65
55
45
35
25
15
5
5
15
04853-011
P
IN
(dBm)
E
RROR (dB)
Figure 11. Distribution of Error at Temperature after Ambient
Normalization vs. Input Amplitude at 2.2 GHz for at least 70 Devices
2.0
2.0
1.6
1.2
0.8
0.4
0
0.4
0.8
1.2
1.6
65
55
45
35
25
15
5
5
15
04853-012
P
IN
(dBm)
E
RROR (dB)
Figure 12. Distribution of Error at Temperature after Ambient
Normalization vs. Input Amplitude at 3.6 GHz for at least 70 Devices
2.0
2.0
1.6
1.2
0.8
0.4
0
0.4
0.8
1.2
1.6
65
55
45
35
25
15
5
5
15
04853-013
P
IN
(dBm)
E
RROR (dB)
Figure 13. Distribution of Error at Temperature after Ambient
Normalization vs. Input Amplitude at 5.8 GHz (T
ADJ
=1000 ) for at least
70 Devices
4.5
4.5
3.6
2.7
1.8
0.9
0
0.9
1.9
2.7
3.6
65
55
45
35
25
15
5
5
04853-014
P
IN
(dBm)
E
RROR (dB)
Figure 14. Distribution of Error at Temperature after Ambient
Normalization vs. Input Amplitude at 8 GHz for at least 70 Devices
AD8318
Rev. 0 | Page 10 of 24
04853-015
j2
j2
j0.5
j0.5
j0.2
j0.2
0
0.2
0.5
1
2
START FREQUENCY = 0.1GHz
STOP FREQUENCY = 8GHz
j1
j1
5.8GHz
8GHz
3.6GHz
2.2GHz
1.9GHz
0.9GHz
0.1GHz
Figure 15. Input Impedance vs. Frequency; No Termination Resistor on INHI
0.07
0
0.01
0.02
0.03
0.04
0.05
0.06
1.4
1.5
1.6
1.7
1.8
04853-
016
V
ENBL
(V)
SUPPLY CURRENT (A)
DECREASING V
ENBL
INCREASING V
ENBL
Figure 16. Supply Current vs. Enable Voltage
04853-017
20ns PER HORIZONTAL DIVISION
VOUT
GND
200mV/VERTICAL
DIVISION
PULSED RF INPUT 0.1GHz,
10dBm
Figure 17. V
OUT
Pulse Response Time. Pulsed RF Input 0.1 GHz, 10 dBm;
C
LPF
= Open
10k
1k
100
10
1
3
10
30
100
300
1k
3k
10k
04853-018
FREQUENCY (kHz)
N
O
ISE SPEC
TR
A
L
D
E
N
S
ITY (
n
V/ H
z
)
40dBm
20dBm
10dBm
0dBm
RF OFF
60dBm
Figure 18. Noise Spectral Density of Output; C
LPF
= Open
1k
100
10
1
3
10
30
100
300
1k
3k
10k
04853-019
FREQUENCY (kHz)
NOISE SPECTRAL DENS
ITY (
n
V/ Hz)
Figure 19. Noise Spectral Density of Output Buffer (from CLPF to VOUT);
C
LPF
= 0.1
F
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
65
55
45
35
25
15
5
5
15
04853-020
P
IN
(dBm)
V
OUT
(V
)
2.0
2.0
1.6
1.2
0.8
0.4
0
0.4
0.8
1.2
1.6
E
RROR (dB)
Figure 20. Output Voltage Stability vs. Supply Voltage at 1.9 GHz When V
P
Varies by 10%, Multiple Devices
AD8318
Rev. 0 | Page 11 of 24
GENERAL DESCRIPTION
The AD8318 is a 9-stage demodulating logarithmic amplifier,
which provides RF measurement and power amplifier control
functions. The design is similar to the AD8313 Logarithmic
Detector/Controller. However, the AD8318 input frequency
range is extended to 8 GHz with 60 dB dynamic range. Other
improvements include: reduced intercept variability versus
temperature, increased dynamic range at higher frequencies, low
noise measurement and controller output (VOUT), adjustable
low-pass corner frequency (CLPF), temperature sensor output
(TEMP), negative transfer function slope for higher accuracy,
and 10 ns response time for RF burst detection capability. A
block diagram is shown in Figure 21.
TEMP
SENSOR
GAIN
BIAS
SLOPE
DET
DET
DET
DET
INHI
INLO
I
V
VOUT
I
V
VSET
CLPF
TEMP
VPSI
ENBL
TADJ
VPSO
CMOP
CMIP
04853-
021
Figure 21. Block Diagram
A fully differential design, using a proprietary high speed
SiGe process, extends high frequency performance. Input INHI
receives the signal with a low frequency impedance of nominally
1200 in parallel with 0.7 pF. The maximum input with 1 dB
log-conformance error is typically 0 dBm (re: 50 ). The noise
spectral density referred to the input is 1.15 nV/Hz, which
is equivalent to a voltage of 118 V rms in a 10.5 GHz band-
width, or a noise power of 66 dBm (re: 50 ). This noise
spectral density sets the lower limit of the dynamic range.
However, the low-end accuracy of the AD8318 is enhanced
by specially shaping the demodulating transfer characteristic
to partially compensate for errors due to internal noise. The
input system common pin, CMIP, provides a quality low
impedance connection to the printed circuit board (PCB)
ground through the use of four package pins. The package
paddle, which is internally connected to the CMIP pin, should
also be grounded to the PCB to reduce thermal impedance from
the die to the PCB.
The logarithmic function is approximated in a piecewise fashion
by 9 cascaded gain stages. (For a more comprehensive expla-
nation of the logarithm approximation, please refer to the
AD8307 data sheet, available at
www.analog.com
.) The cells have
a nominal voltage gain of 8.7 dB each, and a 3 dB bandwidth of
10.5 GHz. Using precision biasing, the gain is stabilized over
temperature and supply variations. Since the cascaded gain
stages are dc-coupled, the overall dc gain is high. An offset
compensation loop is included to correct for offsets within
the cascaded cells. At the output of each of the gain stages, a
square-law detector cell is used to rectify the signal. The RF
signal voltages are converted to a fluctuating differential
current having an average value that increases with signal
level. Along with the nine gain stages and detector cells, an
additional detector is included at the input of the AD8318,
altogether providing a 60 dB dynamic range. After the
detector currents are summed and filtered, the function
I
D
log
10
(V
IN
/V
INTERCEPT
) is formed at the summing node,
where I
D
is the internally set detector current, V
IN
is the
input signal voltage, and V
INTERCEPT
is the intercept voltage
(i.e., when V
IN
= V
INTERCEPT
, the output voltage would be 0 V,
if it were capable of going to 0 V).
AD8318
Rev. 0 | Page 12 of 24
USING THE AD8318
BASIC CONNECTIONS
The AD8318 is specified for operation up to 8 GHz, as a result
low impedance supply pins with adequate isolation between
functions are essential. In the AD8318, the two positive supply
pins, VPSI and VPSO, must be connected to the same potential.
The VPSI pin biases the input circuitry, while the VPSO biases
the low noise output driver for VOUT. Separate commons are
also included in the device. CMOP is used as the common for
the output drivers. All commons should be connected to a low
impedance ground plane.
A power supply voltage of between 4.5 V and 5.5 V should be
applied to VPS0 and VPS1. 100 pF and 0.1 F power supply
decoupling capacitors should be connected close to each power
supply pin. (The two adjacent VPS1 pins can share a pair of
decoupling capacitors because of their proximity.)
0
4853-022
12
CMIP
11
CMIP
10
TADJ
9
VPSO
1
CMIP
2
CMIP
3
VPSI
4
VPSI
13
TEMP
8
CMOP
14
INHI
7
VSET
15
INLO
6
VOUT
16
ENBL
5
CLPF
AD8318
C7
100pF
C8
0.1
F
V
S
C1
1nF
C2
1nF
499
(SEE TEXT)
C6
100pF
C5
0.1
F
V
S
R1
52.3
TEMP
OUT
RF
INPUT
V
S
VOUT
Figure 22. Basic Connections
The paddle of the AD8318's LFCSP package is internally
connected to CMIP. For optimum thermal and electrical
performance, the paddle should be soldered to a low impedance
ground plane.
ENABLE
To enable the AD8318, the ENBL pin must be pulled high.
Taking ENBL low will put the AD8318 in sleep mode, reducing
current consumption to 260 A at ambient. The voltage on
ENBL must be greater than 2 V
BE
(~1.7 V) to enable the device.
When enabled the devices draws less than 1 A. When the ENBL
pin is pulled low, the pin sources 15 A.
The enable interface has high input impedance. A 200 resistor
is placed in series with the ENBL input for added protection.
Figure 23 depicts a simplified schematic of the enable interface.
04853-023
ENBL
CMIP
VPSI
2
V
BE
2
V
BE
DISCHARGE
ENABLE
40k
200
40k
Figure 23. ENBL Interface
INPUT SIGNAL COUPLING
The RF input to the AD8318 (INHI) is single-ended and
must be ac-coupled. INLO (input common) should be
ac-coupled to ground (See Figure 22). Suggested coupling
capacitors are 1 nF ceramic 0402 style capacitors for input
frequencies of 1 MHz to 8 GHz. The coupling capacitors
should be mounted close to the INHI and INLO pins. These
capacitor values can be increased to lower the input stage's
high-pass cutoff frequency. The high-pass corner is set by
the input coupling capacitors and the internal 10 pF high-
pass capacitor. The dc voltage on INHI and INLO will be
about one diode voltage drop below V
PSI
.
The Smith chart in Figure 15 shows the AD8318's input
impedance vs. frequency. Table 4 lists the reflection coeffi-
cient and impedance at select frequencies. For Figure 15 and
Table 4, the 52.3 input termination resistor was removed.
At dc, the resistance is typically 2 k. At frequencies up to 1
GHz, the impedance is approximated as 1000 || 0.7 pF.
The RF input pins are coupled to a network given by the
simplified schematic in Figure 24.
04853-024
VPSI
2k
A = 8.6dB
20k
20k
CURRENT
Gm
STAGE
INLO
INHI
OFFSET
COMP
10pF
10pF
FIRST
GAIN
STAGE
Figure 24. Input Interface
While the input can be reactively matched, in general this is
not necessary. An external 52.3 shunt resistor (connected
on the signal side of the input coupling capacitors, see
Figure 22) combines with the relatively high input imped-
ance to give an adequate broadband 50 match.
AD8318
Rev. 0 | Page 13 of 24
Table 4. Input Impedance for Select Frequency
S11
Frequency
MHz
Real
Imaginary
Impedance
(Series)
100
0.918
-0.041
927-j491
456
0.905
-0.183
173-j430
900
0.834
-0.350
61-j233
1900
0.605
-0.595
28-j117
2200
0.524
-0.616
28-j102
3600
0.070
-0.601
26-j49
5300
-0.369
-0.305
20-j16
5800
-0.326
-0.286
22-j16
8000
-0.390
-0.062
22-j3
OUTPUT INTERFACE
The VOUT pin is driven by a PNP output stage. An internal 10
resistor is placed in series with the emitter follower output and
the VOUT pin. The rise time of the output is limited mainly by
the slew on CLPF. The fall time is an RC limited slew given by
the load capacitance and the pull-down resistance at VOUT.
There is an internal pull-down resistor of 350 . Any resistive
load at VOUT is placed in parallel with the internal pull-down
resistor and provides additional discharge current.
04853-025
+
0.2V
150
200
10
VOUT
VPSO
CLPF
CMOP
Figure 25. Output Interface
SETPOINT INTERFACE
The V
SET
input drives the high impedance (250 k) input of an
internal op amp. The V
SET
voltage appears across the internal
3.13 k resistor to generate I
SET
. When a portion of V
OUT
is
applied to VSET, the feedback loop forces -I
D
log
10
(V
IN
/V
INTERCEPT
) = I
SET
. If V
SET
= V
OUT
/X, then I
SET
=
V
OUT
/(X 3.13 k). The result is
V
OUT
= (-I
D
3.13 k X) log
10
(V
IN
/V
INTERCEPT
)
04853-026
3.13k
I
SET
CMOP
VSET
Figure 26. VSET Interface
The slope is given by I
D
X 3.13 k = 500 mV X. For
example, if a resistor divider to ground is used to generate a
V
SET
voltage of V
OUT
/2, then X = 2. The slope will be set to
1 V/decade or 50 mV/dB.
TEMPERATURE COMPENSATION OF OUTPUT
VOLTAGE
The AD8318 functionality includes the capability to
externally trim the temperature drift. Attaching a ground-
referenced resistor to the T
ADJ
pin alters an internal current,
which works to minimize intercept drift vs. temperature. As
a result, the T
ADJ
resistor can be optimized for operation at
different frequencies.
04853-027
2k
I
COMP
~0.4V
TADJ
2V
INTERNAL
V
Figure 27. TADJ Interface
A resistor, nominally 500 for optimal temperature
compensation at 2.2 GHz input frequency, is connected
between this pin and ground (see Figure 22). The value of
this resistor partially determines the magnitude of an analog
correction coefficient, which is employed to reduce
intercept drift.
Table 5 lists recommended resistors for other frequencies.
These resistors have been chosen to provide the best overall
temperature drift based on measurements of a diverse
population of devices.
The relationship between output temperature drift and
frequency is not linear and cannot be easily modeled. As a
result, experimentation is required to choose the correct
T
ADJ
resistor at frequencies not listed in Table 5.
AD8318
Rev. 0 | Page 14 of 24
Table 5. Recommended T
ADJ
Resistors
Frequency Recommended
T
ADJ
900 MHz
500
1.9 MHz
500
2.2 GHz
500
3.6 GHz
51
5.8 GHz
1 k
8 GHz
500
TEMPERATURE SENSOR
The AD8318 internally generates a voltage that is proportional-
to-absolute-temperature (V
PTAT
). The V
PTAT
voltage is multiplied
by a factor of 5, resulting in a +2 mV/C output at the TEMP pin.
The output voltage at 27C is typically 600 mV. An emitter
follower drives the TEMP pin, as shown in Figure 28.
04853-028
1k
4k
CMIP
INTERNAL
VPSI
TEMP
Figure 28. Temp Sensor Interface
The internal pull-down resistance is 5 k. The temperature
sensor has a slope of +2 mV/C.
The temp sensor output will vary with output current due to
increased die temperature. Output loads less than 1 k will draw
enough current from the output stage causing this increase to
occur. An output current of 10 mA will result in the voltage on
the temp sensor to increase by 1.5C, or ~3 mV.
To get the best precision from the temperature sensor, ensure
that supply current to AD8318 remains fairly constant (i.e., no
heavy load drive).
MEASUREMENT MODE
When the V
OUT
voltage or a portion of the V
OUT
voltage is fed
back to VSET, the device operates in measurement mode. As
seen in Figure 29, the AD8318 has an offset voltage, a negative
slope, and a V
OUT
measurement intercept greater than its input
signal range.
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
2.5
2.0
1.5
1.0
0.5
0
0.5
1.0
1.5
2.0
65 60 55 50 45 40 35 30 25 20 15 10 5
0
5
04853-029
P
IN
(dBm)
INTERCEPT
ERROR (dB)
V
OU
T
(V)
ERROR 25
C
V
OUT
25
C
RANGE FOR CALCULATION
OF SLOPE AND INTERCEPT
Figure 29. Typical Output Voltage vs. Input Signal
The output voltage versus input signal voltage of the
AD8318 is linear-in-dB over a multidecade range. The
equation for this function is of the form
V
OUT
= X V
SLOPE/DEC
log
10
(V
IN
/V
INTERCEPT
) (1)
= X V
SLOPE/dB
20 log
10
(V
IN
/V
INTERCEPT
) (2)
where:
X is the feedback factor in V
SET
= V
OUT
/X
V
INTERCEPT
is expressed in V
rms
.
V
SLOPE/DEC
is nominally 500 mV/decade or -25 mV/dB.
V
INTERCEPT
expressed in dBV is the x-axis intercept of the
linear-in-dB transfer function shown in Figure 29.
V
INTERCEPT
is +7 dBV (+20 dBm, re: 50 or 2.239 V
rms
)
for a
sinusoidal input signal.
The slope of the transfer function can be increased to
accommodate various converter mV per dB (LSB per dB)
requirements. However, increasing the slope may reduce
the dynamic range. This is due to the limitation of the
minimum and maximum output voltages, determined by
the chosen scaling factor X.
The minimum value for V
OUT
is X V
OFFSET
. An offset
voltage, V
OFFSET
, of 0.5 V is internally added to the detector
signal.
V
OUT(MIN)
= (X V
OFFSET
)
The maximum output voltage is 2.1 V X, and cannot
exceed 400 mV below the positive supply.
AD8318
Rev. 0 | Page 15 of 24
V
OUT(MAX)
= (2.1 V X) when X < (V
P
400 mV)/(2.1 V)
V
OUT(MAX)
= (V
P
400 mV) when X (V
P
400 mV)/(2.1 V)
When X = 1, the typical output voltage swing is 0.5 V to 2.1 V.
The output voltage swing can be modeled by using the equations
above and restricted by the following equation:
V
OUT(MIN)
< V
OUT
< V
OUT(MAX)
For the case when X = 4 and V
P
= 5 V
(X V
OFFSET
) < V
OUT
< (V
P
400 mV)
(4 0.5 V) < V
OUT
< (2.1 V 4)
2 V < V
OUT
< 4.6 V
For X = 4, Slope = -100 mV/dB; V
OUT
can swing 2.6 V, and
usable dynamic range will be reduced to 26 dB from 0 dBm to
26 dBm.
The slope is very stable versus process and temperature
variation. When base-10 logarithms are used, V
SLOPE/DECADE
represents the "volts/decade." A decade corresponds to 20 dB,
V
SLOPE/DECADE
/20 = V
SLOPE/dB
represents the slope in "volts/dB."
As noted in the equations above, the V
OUT
voltage has a negative
slope. This is also the correct slope polarity to control the gain of
many power amplifiers and other VGAs in a negative feedback
configuration. Since both the slope and intercept vary slightly
with frequency, it is recommended to refer to the specification
pages for application specific values for slope and intercept.
Although demodulating log amps respond to input signal
voltage, not input signal power, it is customary to discuss the
amplitude of high frequency signals in terms of power. In this
case, the characteristic impedance of the system, Z
o
, must be
known to convert voltages to their corresponding power levels.
Starting with the definitions of dBm and dBV,
P(dBm) = 10 log
10
(V
rms
2
/(Z
O
1 mW)) (3)
V(dBV) = 20 log
10
(V
rms
/1 V
rms
) (4)
Expanding Equation 3 gives us:
P(dBm) = 20 log
10
(V
rms
) - 10 log
10
( Z
O
1 mW) (5)
and given Equation 4, we can rewrite Equation 5 as
P(dBm) = V(dBV) - 10 log
10
(Z
O
1 mW) (6)
For example, P
INTERCEPT
for a sinusoidal input signal
expressed in terms of dBm (decibels referred to 1 mW), in a
50 system is:
P
INTERCEPT
(dBm) = V
INTERCEPT
(dBV)
10 log
10
(Zo 1 mW) (7)
= +7 dBV - 10 log
10
(50 10
-3
) = +20 dBm
Further information on the intercept variation dependence
upon waveform can be found in the AD8313 and AD8307
data sheets.
AD8318 data sheet specifications for slope and intercept
have been calculated based on a best straight line fit using
measured data in the -10 dBm to -50 dBm range (see
Figure 29).
DEVICE CALIBRATION AND ERROR
CALCULATION
The measured transfer function of the AD8318 at
2.2 GHz is shown in
Figure 30
. The figure shows plots of
both output voltage versus input power and calculated
error versus input power.
As the input power varies from
-
65 dBm to 0 dBm, the
output voltage varies from 2 V to about 0.5 V.
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
2.5
2.0
1.5
1.0
0.5
0
0.5
1.0
1.5
2.0
65 60 55
45 40 35 30 25 20 15
5
0
5
04853-
030
P
IN
(dBm)
INTERCEPT
PIN
1
PIN
2
V
OUT
(V
)
ERROR (
d
B)
V
OUT
+25
C
V
OUT
40
C
V
OUT
+85
C
ERROR +25
C
ERROR 40
C
ERROR +85
C
VOUT
2
VOUT
1
VOUT
IDEAL
= SLOPE
(P
IN
INTERCEPT)
SLOPE = (VOUT
1
VOUT
2
)/(PIN
1
PIN
2
)
INTERCEPT = PIN
1
(VOUT
1
/SLOPE)
ERROR (dB) = (VOUT
VOUT
IDEAL
)/SLOPE
Figure 30. Transfer Function at 2.2 GHz
Because slope and intercept vary from device to device,
board-level calibration must be performed to achieve high
accuracy.
We can rewrite the equation for output voltage from the
previous section using an intercept expressed in dBm
V
OUT
= Slope (P
IN
Intercept) (8)
AD8318
Rev. 0 | Page 16 of 24
In general, the calibration is performed by applying two known
signal levels to the AD8318's input and measuring the
corresponding output voltages. The calibration points are
generally chosen to be within the linear-in-dB operating range of
the device (see Figure 30). Calculation of slope and intercept is
done using the equations
Slope = (V
OUT1
- V
OUT2
)/(P
IN1
P
IN2
) (9)
Intercept = P
IN1
- V
OUT1
/Slope (10)
Once Slope and Intercept have been calculated, an equation can
be written which will allow calculation of an (unknown) input
power based on the output voltage of the detector.
P
IN
(unknown) = V
OUT
(measured)/Slope + Intercept (11)
Using the equation for the ideal output voltage (7) as a reference,
the log conformance error of the measured data can be
calculated:
Error(dB) = (V
OUT(MEASURED)
- V
OUT(IDEAL)
)/Slope (12)
Figure 30 includes a plot of the error at 25C, the temperature at
which the log amp is calibrated. Note that the error is not zero.
This is because the log amp does not perfectly follow the ideal
V
OUT
versus P
IN
equation, even within its operating region. The
error at the calibration points (-12 dBm and -52 dBm in this
case) will, however, be equal to zero by definition.
Figure 30 also includes error plots for the output voltage at
-40C and +85 C. These error plots are calculated using the
slope and intercept at 25C. This is consistent with calibration in
a mass-production environment where calibration at
temperature is not practical.
SELECTING CALIBRATION POINTS TO IMPROVE
ACCURACY OVER A REDUCED RANGE
In some applications very high accuracy is required at just one
power level or over a reduced input range. For example, in a
wireless transmitter, the accuracy of the high power amplifier
(HPA) will be most critical at or close to full power.
Figure 31 shows the same measured data as Figure 30. Notice
that accuracy is very high from -10 dBm to -30 dBm. Below
-30 dBm the error increases to about -1 dB. This is because the
calibration points have been changed to -14 dBm and -26 dBm.
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.6
0.4
0.2
2.5
2.0
1.5
1.0
0.5
0
0.5
1.0
1.5
2.0
2.5
65 60 55
45 40 35 30
20
10 5
0
5
04853-031
P
IN
(dBm)
PIN
1
PIN
2
V
OUT
(V
)
E
RROR (d
B)
ERROR +25
C
ERROR 40
C
ERROR +85
C
V
OUT
+25
C
V
OUT
40
C
V
OUT
+85
C
VOUT
2
VOUT
1
Figure 31. Output Voltage and Error vs. P
IN
with 2-Point Calibration at
10 dBm and 30 dBm
Calibration points should be chosen to suit the application
at hand. In general, though, the calibration points should
never be chosen in the nonlinear portion of the log amp's
transfer function (above -5 dBm or below -60 dBm in this
case).
Figure 32 shows how calibration points can be adjusted to
increase dynamic range, but at the expense of linearity. In
this case the calibration points for slope and intercept are set
at -4 dBm and -60 dBm. These points are at the end of the
device's linear range. Once again at 25C, we see an error of
0 dB at the calibration points. Note also that the range over
which the AD8318 maintains an error of < 1 dB is
extended to 60 dB at 25C and 58 dB over temperature. The
disadvantage of this approach is that linearity suffers,
especially at the top end of the input range.
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
65 60 55 50 45 40 35 30 25 20 15 10 5
0
5
04853-038
P
IN
(dBm)
V
OUT
(V
)
2.5
2.5
2.0
1.5
1.0
0.5
0
0.5
1.0
1.5
2.0
E
RROR (dB)
V
OUT
+25
C
V
OUT
40
C
V
OUT
+85
C
ERROR +25
C
ERROR 40
C
ERROR +85
C
58dB DYNAMIC RANGE (
1dB ERROR)
Figure 32. Dynamic Range Extension by Choosing Calibration Points
that are Close to the End of the Linear Range
Another way of presenting the error function of a log amp
detector is shown in Figure 33. In this case, the dB error at
hot and cold temperatures is calculated with respect to the
AD8318
Rev. 0 | Page 17 of 24
output voltage at ambient. This is a key difference in comparison
to the previous plots. Up to now, all errors have been calculated
with respect to the ideal transfer function at ambient.
When we use this alternative technique, the error at ambient
becomes by definition equal to 0 (see Figure 33)
.
This would be valid if the device transfer function perfectly
followed the ideal V
OUT
= Slope (Pin-Intercept) equation.
However since a log amp in practice will never perfectly follow
this equation (especially outside of its linear operating range),
this plot tends to artificially improve linearity and extend the
dynamic range. This plot is a useful tool for estimating
temperature drift at a particular power level with respect to the
(non-ideal) output voltage at ambient. However, to achieve this
level of accuracy in an end application would require calibration
at multiple points in the device's operating range.
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
65 60 55 50 45 40 35 30 25 20 15 10 5
0
5
04853-032
P
IN
(dBm)
V
OUT
(V
)
2.5
2.5
2.0
1.5
1.0
0.5
0
0.5
1.0
1.5
2.0
E
RROR (dB)
V
OUT
+25
C
V
OUT
40
C
V
OUT
+85
C
ERROR +25
C wrt V
OUT
ERROR 40
C wrt V
OUT
ERROR +85
C wrt V
OUT
Figure 33. Error vs. Temperature with respect to Output Voltage at 25 C Does
Not Take into Account Transfer Functions' Nonlinearities at 25C
VARIATION IN TEMPERATURE DRIFT FROM DEVICE
TO DEVICE
Figure 34 shows a plot of output voltage and error for multiple
AD8318 devices, measured in this case at 5.8 GHz. The
concentration of black error plots represents the performance
of the population at 25C (slope and intercept has been
calculated for each device). The red and blue plots of error
indicate the measured behavior of a population of devices over
temperature. This suggests a range on the drift (from device to
device) of 1.2 dB.
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
65
55
45
35
25
15
5
5
15
04853-050
P
IN
(dBm)
V
OUT
(V
)
2.0
2.0
1.6
1.2
0.8
0.4
0
0.4
0.8
1.2
1.6
E
RROR (dB)
Figure 34. Output Voltage and Error vs. Temperature (+25C, 40C, and
+85C) of a Population of Devices Measured at 5.8 GHz
TEMPERATURE DRIFT AT DIFFERENT
TEMPERATURES
Figure 35 shows the log slope and error over temperature
for a 5.8 GHz input signal. Error due to drift over
temperature consistently remains within 0.5 dB, and only
begins to exceed this limit when the ambient temperature
drops below -20C. For all frequencies when using a
reduced temperature range higher measurement accuracy is
achievable.
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
65 60 55 50 45 40 35 30 25 20 15 10 5
0
5
04853-039
P
IN
(dBm)
OUT
2.5
2.5
2.0
1.5
1.0
0.5
0
0.5
1.0
1.5
2.0
E
RROR (dB)
V
(V
)
VAPC +25C
VAPC 0C
ERROR 10C
VAPC 40C
VAPC +70C
ERROR 20C
VAPC +85C
ERROR +25C
ERROR 0C
VAPC 20C
ERROR +85C
VAPC 10C
ERROR 40C
ERROR +70C
Figure 35. Typical Drift at 5.8 GHz for Various Temperatures
SETTING THE OUTPUT SLOPE IN
MEASUREMENT MODE
To operate in measurement mode, VOUT must be
connected to VSET. This yields the nominal logarithmic
slope of approximately -25 mV/dB. The output swing
corresponding to the specified input range will then be
approximately 0.5 V to 2.1 V. The slope and output swing
AD8318
Rev. 0 | Page 18 of 24
can be increased by placing a resistor divider between VOUT
and VSET (i.e., one resistor from VOUT to VSET and one
resistor from VSET to common). For example, if two equal
resistors are used (e.g., 10 k/10 k), the slope will double to
approximately -50 mV/dB. The input impedance of VSET is
approximately 500 k. Slope setting resistors should be kept
below ~50 k to prevent this input impedance from affecting
the resulting slope. When increasing the slope, the new output
voltage range cannot exceed the output voltage swing capability
of the output stage. Refer to the Measurement Mode section of
the data sheet.
04853-033
VOUT
AD8318
50mV/dB
VSET
10k
10k
Figure 36. Increasing the Slope
RESPONSE TIME CAPABILITY
The AD8318 has a 10 ns rise/fall time capability (10% 90%) for
input power switching between the noise floor and
0 dBm. This capability enables RF burst measurements at
repetition rates to beyond 60 MHz. In most measurement
applications, the AD8318 will have an external capacitor
connected to CLPF to provide additional filtering for VOUT.
However, the use of the CLPF capacitor slows the response time
as does stray capacitance on VOUT. For an application requiring
maximum RF burst detection capability, the CLPF capacitor pin
should be left unconnected. In this case, the integration function
is provided by the 700 fF on-chip capacitor.
There is a 10 internal resistor in series with the output driver,
an external 40 back-terminating resistor should be added in
series at the output when driving a 50 coaxial cable. The back-
terminating resistor should be placed close to the VOUT pin.
The AD8318 has the drive capability to drive a 50 load at the
end of the coaxial cable or transmission line when back
terminated. See Figure 37.
The circuit diagram in Figure 37 shows the AD8318 used with a
high speed comparator circuit. The 40 series resistor at the
output of the AD8318 combines with an internal 10 to
properly match to the 50 input of the comparator.
04853-040
PULSED RF
INPUT
1nF
+5V
40
100
100
50
50
1nF
52.3
INHI
INLO
VOUT
VPOS
GND
AD8318
VSET
ADCMP563
+5V
5.2V
5.2V
V
REF
= 1.8V1.2V
50
50
COMPARATOR
OUTPUT
AD8318
OUTPUT
Figure 37. AD8318 Operating with the High Speed ADCMP563 Comparator
04853-041
0
100
200
300
400
500
600
700
800
TIME (ns)
AD8318
OUTPUT
COMPARATOR
OUTPUT
PULSED RF
INPUT
50dB
30dB
20dB
10dB
Figure 38. Pulse Response of AD8318 and Comparator for RF Pulses of
Varying Amplitudes
Figure 38 shows the response of the AD8318 and the
comparator for a 500 MHz pulsed sine wave of varying
amplitudes. The output level of the AD8318 is the signal
strength of the input signal. For applications where these RF
bursts are very small, the output level will not change by a
large amount. Using a comparator is beneficial because it
will turn the output of the log amp into a limiter-like signal.
CONTROLLER MODE
The AD8318 provides a controller mode feature at the
VOUT pin. Using V
SET
for the setpoint voltage, it is possible
for the AD8318 to control subsystems, such as power
amplifiers (PAs), variable gain amplifiers (VGAs), or
variable voltage attenuators (VVAs) that have output power
that increases monotonically with respect to their gain
control signal.
To operate in controller mode, the link between VSET and
VOUT is broken. A setpoint voltage is applied to the VSET
input; VOUT is connected to the gain control terminal of
the VGA and the detector's RF input is connected to the
output of the VGA (usually using a directional coupler and
some additional attenuation). Based on the defined
relationship between V
OUT
and the RF input signal when the
device is in measurement mode, the AD8318 will adjust the
voltage on VOUT (VOUT is now an error amplifier output)
until the level at the RF input corresponds to the applied
V
SET
. When the AD8318 operates in controller mode, there
is no defined relationship between V
SET
and V
OUT
voltage;
V
OUT
will settle to a value that results in the correct input
signal level appearing at INHI/INLO.
In order for this output power control loop to be stable,
a ground-referenced capacitor must be connected to the
C
FLT
pin.
This capacitor integrates the error signal (which is actually a
current) that is present when the loop is not balanced.
AD8318
Rev. 0 | Page 19 of 24
04853-034
RFIN
VGA/VVA
GAIN
CONTROL
VOLTAGE
DIRECTIONAL
COUPLER
ATTENUATOR
INHI
VSET
INLO
CLPF
VOUT
AD8318
52.3
1nF
C
FLT
1nF
DAC
Figure 39. AD8318 Controller Mode
Decreasing V
SET
, which corresponds to demanding a higher
signal from the VGA, will tend to increase V
OUT
. The gain
control voltage of the VGA must have a positive sense that is
increasing gain control voltage increases gain.
The basic connections for operating the AD8318 as an analog
controller with the AD8367 are shown in Figure 40. The AD8367
is a low frequency to 500 MHz VGA with 45 dB of dynamic
range. This configuration is very similar to the one shown in
Figure 39.
The gain of the AD8367 is controlled by the voltage applied to
the GAIN pin. This voltage, V
GAIN
, is scaled linear-in-dB with a
slope of 20 mV/dB and runs from 50 mV at 2.5 dB of gain, up
to 1.0 V at +42.5 dB.
The incoming RF signal to the AD8367 has a varying amplitude
level; receiving and demodulating it with the lowest possible
error requires that the signal levels be optimized for the highest
signal-to-noise ratio (SNR) feeding into the analog-to-digital
converters (ADC). This can be accomplished by using an
automatic gain control (AGC) loop. In Figure 40 the voltage
output of the AD8318 is used to modify the gain of the AD8367
until the incoming RF signal produces an output voltage that is
equal to the setpoint voltage V
SET
.
04853-047
+3V
INPT
VOUT
VPOS GND
GAIN
AD8367
VGA
HPLF
RF INPUT SIGNAL
RF OUTPUT SIGNAL
+5V
VSET
CLPF
INHI
VPOS
GND
AD8318
VOUT
INLO
R1
1k
1nF
1nF
0.1
F
C
FLT
100pF
100MHz
BANDPASS
FILTER
174
R2
261
R
HP
100
C
HP
100pF
DAC
+V
SET
SETPOINT
VOLTAGE
57.6
Figure 40. AD8318 Operating in Controller Mode to Provide Automatic Gain
Control Functionality in Combination with the AD8367
This AGC loop is capable of controlling signals over ~45 dB
dynamic range. The output of the AD8367 is designed to
drive loads 200 . As a result, it is not necessary to use the
53.6 resistor at the input of the AD8318; the nominal
input impedance of 2 k is sufficient. If the AD8367's
output is to be driving a 50 load, such as an oscilloscope
or spectrum analyzer, a simple resistive divider network can
be used. Note that the divider used in Figure 40 has an
insertion loss of 11.5 dB.
Figure 41 shows the transfer function of output power
versus V
SET
voltage for a 100 MHz sine wave at -40 dBm
into the AD8367.
0
60
55
50
45
40
35
30
25
20
15
10
5
1.2
1.2
1.0
0.8
0.6
0.4
0.2
0
0.2
0.4
0.6
0.8
1.0
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
04853-
048
V
SET
(V)
ERROR (dB)
P
OUT
(d
Bm)
Figure 41. AD8367 Output Power vs. AD8318 Setpoint Voltage
In order for the AGC loop to remain locked, the AD8318
must track the envelope of the VGA's output signal and
provide the necessary voltage levels to the AD8367's gain
control input. Figure 42 shows an oscilloscope screenshot of
the AGC loop depicted in Figure 40. A 50 MHz sine wave
with 50% AM modulation is applied to the AD8367. The
output signal from the VGA is a constant envelope sine
wave with an amplitude corresponding to a setpoint voltage
at the AD8318 of 1.0 V.
Figure 42. Oscilloscope Screenshot Showing an AM Modulated Input
Signal to the AD8367. The AD8318 tracks the envelope of this input
signal and applies the appropriate voltage to ensure a constant output
from the AD8367.
AD8318
Rev. 0 | Page 20 of 24
The 45 dB control range is constant for the range of V
SET
voltages. The input power levels to the AD8367 must be
optimized to achieve this range. In Figure 43 the minimum and
maximum input power levels are shown vs. setpoint voltage.
10
80
70
60
50
40
30
20
10
0
0.5
0.6
0.7
0.8
0.9
1.0
1.2
1.2
1.3
1.4
1.5
04853-049
V
SET
(V)
P
IN
(dBm)
MAXIMUM INPUT LEVEL
MINIMUM INPUT LEVEL
Figure 43. Setpoint Voltage vs. Input Power. Optimal signal levels must be
used to achieve the full 45 dB dynamic range capabilities of the AD8367.
In some cases, it may be found that if V
GAIN
is >1.0 V it may take
an unusually long time for the AGC loop to recover; that is, the
output of the AD8318 will remain at an abnormally high value
and the gain will be set to its maximum level. A voltage divider is
placed between the output of the AD8318 and the AD8367's
GAIN pin to ensure that V
GAIN
will not exceed 1.0 V.
In Figure 40, C
HP
and R
HP
are configured to reduce oscillation
and distortion due to harmonics at higher gain settings. Some
additional filtering is recommended between the output of the
AD8367 and the input of the AD8318. This will help to decrease
the output noise of the AD8367, which may reduce the dynamic
range of the loop at higher gain settings (smaller V
SET
).
Response time and the amount of signal integration are
controlled by C
FLT
--this functionality is analogous to the
feedback capacitor around an integrating amplifier. While it is
possible to use large capacitors for C
FLT
, in most applications
values under 1 nF will provide sufficient filtering.
Calibration in controller mode is similar to the method used in
measurement mode. A simple two-point calibration can be done
by applying two known V
SET
voltages or DAC codes and
measuring the output power from the VGA. Slope and intercept
can then be calculated with the following equations.
Slope = (V
SET1
- V
SET2
)/(P
OUT1
- P
OUT2
) (13)
Intercept = P
OUT1
- V
SET1
/Slope (14)
V
SET
= Slope (Px - Intercept) (15)
More information on AGC applications can be found in the
AD8367 Data Sheet.
CHARACTERIZATION SETUPS AND METHODS
The general hardware configuration used for the AD8318
characterization is shown in Figure 45. The primary setup
used for characterization was measurement mode. The
characterization board is similar to the customer evaluation
board with the exception that the RFIN had a Rosenberger
SMA connector and R10 was changed to a 1 k resistor to
remove cable capacitance from the bench characterization
setup. Slope and intercept were calculated using linear
regression from -50 dBm to -10 dBm. The slope and
intercept are used to generate an ideal line. Log conform-
ance error is the difference from the ideal line and the
measured output voltage for a given temperature in dB. For
additional information on the error calculation, refer to the
Device Calibration and Error Calculation section.
The hardware configuration for pulse response measure-
ment replaced the 0 series resistor on the VOUT pin with
a 40 resistor and the CLPF pin was left open. Pulse
response time was measured using a Tektronix TDS51504
Digital Phosphor Oscilloscope. Both channels on the scope
had 50 termination selected. The 10 internal to the
output interface and the 40 series resistor attenuate the
output response by 2. RF input frequency was 100 MHz
with -10 dBm at the input of the device. The RF burst was
generated using SMT06 with the pulse option with a period
of 1.5 S, a width of 0.1 S, and a pulse delay of 0.04 S. The
output response was triggered using the video out from the
SMT06. Refer to Figure 44 for an overview of the test setup.
04853-
046
VIDEO
OUT
RF OUT
7dBm
R AND S SMT06
TEKTRONIX
TDS51504
CH1* CH3* TRIGGER
VOUT
GND
5V
AD8318
VPOS
VSET
INHI
INLO
40
52.3
1nF
1nF
*50
TERMINATION
3dB
SPLITTER
Figure 44. Pulse Response Measurement Test Setup
To measure noise spectral density, the evaluation replaced
the 0 resistor in series with the VOUT pin with a 1 F dc
blocking capacitor. The capacitor was used because the
FSEA cannot handle dc voltages at the RF input. The CLPF
pin was left open for data collected for Figure 18. For
Figure 19 a 1 F capacitor was placed between CLPF and
ground. The large capacitor filtered the noise from the
detector stages of the log amp. Noise spectral density
measurements were made using R&S spectrum analyzer
FSEA and R&S SMT06 signal generator. The signal
generator's frequency was set to 2.2 GHz. The spectrum
analyzer had a span of 10 Hz, resolution bandwidth of
50 Hz, video bandwidth of 50 Hz, and averaged the signal
100 times. Data was adjusted to account for the dc blocking
capacitor impedance on the output at lower frequencies.
AD8318
Rev. 0| Page 21 of 24
EVALUATION BOARD
Table 6. Evaluation Board (Rev A) Configuration Options
Component Function
Default
Conditions
TP1, TP2
Supply and Ground Connections
Not Applicable
SW1
Device Enable: When in position A, the ENBL pin is connected to VP and the
AD8318 is in operating mode. In position B, the ENBL pin is grounded
through R3, putting the device in power-down mode. The
ENBL pin may be
exercised by a pulse generator connected to J3 with SW1 in position B.
SW1 = A
R3 = 10k (Size 0603)
R1, C1, C2
Input Interface: The 52.3
resistor in position R1 combines with the
AD8318's internal input impedance to give a broadband input impedance
of around 50
. Capacitors C1 and C2 are DC blocking capacitors. A reactive
impedance match can be implemented by replacing R1 with an inductor
and C1 and C2 with appropriately-valued capacitors .
R1 = 52.3
(Size 0402)
C1 = 1 nF (Size 0402)
C2 = 1 nF (Size 0402)
R2
Temperature Sensor Interface: The temperature sensor output voltage is
available at J1, via the current limiting resistor, R2.
C4
Temperature Compensation Interface: The internal temperature
compensation resistor is optimized for an input signal of 2.2 GHz when C4 is
1 k
. This circuit can be adjusted to optimize performance for other input
frequencies by changing the value of the resistor in position C4. Note that
the designation C4 on the evaluation board is a typographical error as this
pad will always be populated with a resistor. This error will be corrected on
the Rev B revision of the board.
C4 = 500 k
(Size 0603)
R7, R8, R9, R10
Output Interface--Measurement Mode: In measurement mode, a portion of
the output voltage is fed back to pin VSET via R7. The magnitude of the
slope of the VOUT output voltage response may be increased by reducing
the portion of VOUT that is fed back to VSET. R10 can be used as a back-
terminating resistor or as part of a single-pole low-pass filter.
R7 = 0
= (Size 0402)
R8 = open (Size 0402)
R9 = open (Size 0402
R10= 0
(Size 0402)
R7, R8, R9, R10
Output Interface--Controller Mode: In this mode, R7 must be open. In
controller mode, the AD8318 can control the gain of an external
component. A setpoint voltage is applied to pin VSET, the value of which
corresponds to the desired RF input signal level applied to the AD8318 RF
input. A sample of the RF output signal from this variable-gain component is
selected, typically via a directional coupler, and applied to AD8318 RF input.
The voltage at pin VOUT is applied to the gain control of the variable gain
element. A control voltage is applied to pin VSET via R9 and R8. The
magnitude of the control voltage may optionally be attenuated via the
voltage divider comprised of R8 and R9, or a capacitor may be installed in
position R8 to form a low-pass filter along with R9.
R7 = open (Size 0402)
R8 = open (Size 0402)
R9 = 0
(Size 0402)
R10 = 0
(Size 0402)
C5, C6, C7, C8, R5,
R6
Power Supply Decoupling: The nominal supply decoupling consists of a
100 pF filter capacitor placed physically close to the AD8318, a 0
series
resistor and a 0.1 F capacitor placed nearer to the power supply input pin.
C6 = 100 pF (Size 0402)
C7 = 100 pF (Size 0402)
C5 = 0.1 F (Size 0603)
C8 = 0.1 F (Size 0603)
R5 = 0
(Size 0603)
R6 = 0
(Size 0603)
C9
Filter Capacitor: The low-pass corner frequency of the circuit that drives pin
VOUT can be lowered by placing a capacitor between CLPF and ground.
C4 = open (Size 0603)
AD8318
Rev. 0 | Page 22 of 24
04853-035
12
CMIP
11
CMIP
10
TADJ
9
VPSO
1
CMIP
2
CMIP
3
VPSI
4
VPSI
13
TEMP
8
CMOP
14
INHI
7
VSET
15
INLO
6
VOUT
16
ENBL
5
CLPF
AD8318
C7
100pF
C9
OPEN
C8
0.1
F
V
S
TP1
VP
C1 1nF
C2 1nF
C4
499
(SEE TEXT)
C6
100pF
C5
0.1
F
V
S
R5
0
R1
52.3
V
S
SW1
R3
10k
TP2
GND
J5
VSET
J3
ENBL
J2
INHI
J1
TEMP
J4
VOUT
R7
0
R9
OPEN
R10
0
R8
OPEN
R2
1k
R6
0
Figure 45. Evaluation Board Schematic (Rev A)
04853-036
Figure 46. Component Side Layout
04853-037
Figure 47. Component Side Silkscreen
AD8318
Rev. 0| Page 23 of 24
OUTLINE DIMENSIONS
16
5
13
8
9
12
1
4
2.25
2.10 SQ
1.95
0.75
0.60
0.50
0.65 BSC
1.95 BSC
0.35
0.28
0.25
12 MAX
0.20 REF
SEATING
PLANE
PIN 1
INDICATOR
TOP
VIEW
4.0
BSC SQ
3.75
BSC SQ
0.60 MAX
0.60 MAX
0.05 MAX
0.02 NOM
0.80 MAX
0.65 TYP
PIN 1
INDICATOR
1.00
0.85
0.80
COPLANARITY
0.08
0.25 MIN
EXPOSED
PAD
(BOTTOM VIEW)
COMPLIANT TO JEDEC STANDARDS MO-220-VGGC
Figure 48. 16-Lead Lead Frame Chip Scale Package [LFCSP]
(CP-16)
Dimensions shown in millimeters
ORDERING GUIDE
AD8318 Products
Temperature Package
Package Description
Package Outline
Ordering Qnty.
AD8318ACPZ-REEL7
1
40C to +85C
16-Lead LFCSP
CP-16
1500
AD8318ACPZ-WP
1, 2
40C to +85C
16-Lead LFCSP
CP-16
64
AD8318-EVAL
Evaluation
Board
1
Z = Pb-free part.
2
WP = Waffle Pack.
AD8318
Rev. 0 | Page 24 of 24
NOTES
2004 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D0485307/04(0)
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